Many developmental events during embryogenesis and regulatory events in adults depend upon the presence of extracellular signalling polypeptides (ESPs). Over 35 different ESPs exert their immediate effect through the activation of a set of latent transcription factors called STATs, dual function proteins that serve as signal transducers and activators of transcription. The array of physiologic events controlled at least in part by the STATs include innate immunity, i.e. the initial response to invading bacteria and viruses, proper T cell function particularly in choosing the type of immune responses to invading organisms and many other functions in the bone marrow. Developmental events such as proper epithelial cell development in breast tissue and correct responses to growth hormone also depend on these proteins. Finally, proper growth control requires the action of these proteins. This project has the central long-term goal of understanding the molecular basis for the nuclear action of the STATs in changing transcription rates. We will complete our earlier studies on the definition of transcriptional activation domains, TADs, of Stats 1, 2 and 3 by testing in in vivo transcriptional and cell biologic assays the specificity of each TAD. Proteins that are specifically interactive with the -COOH terminal TAD of Stat1, a domain already demonstrated to be required in transcriptional activation have been detected. The identification of each of these TAD-interactive proteins through mass spectrographic identification by peptide content or by cloning genes of previously unrecognized proteins will be a major initial goal. These interactive proteins very likely include co-activators that may be specific for the STAT group of transcription factors. In vivo and in vitro transcriptional assays of the role of these Stat1-TAD interactive proteins will then be carried out. The studies of functional interaction of the Stat 1, 2 and 3 interactive proteins should provide comprehensive insight into how STAT proteins change gene transcription to induce or maintain the specific phenotypic properties associated with the many extracellular signalling polypeptides.